1. Field of the Invention
This invention concerns a tunnel drier incorporating a screw conveyor particularly for plastic materials being treated in a recovery and regeneration line for waste plastic.
2. Description of the Prior Art
It is known that thermoplastic materials like PVC, polyethylene, polypropylene, etc. for various uses, such as shopping bags, packing sacks, greenhouse awnings, etc., are recovered after use to be recycled to prepare new products. To this end the waste is run through a series of shearing, washing and regranulation operations to prepare a material in granular form for extrusion or injection. After shearing and washing, and before densification, the washed material must be dried. Currently, this operation is effected in tunnel driers generally powered by diesel fuel, electricity or steam. In all these cases, a large amount of energy is expended for the operation. If oil is used, a supply system must also be installed with a tank, tubes and pump, which of course then require maintenance. Also, if steam is used, a supplementary system is required with a steam generator and distribution network, also requiring maintenance. If electricity is used, costs are extremely high to satisfy the power requirements for the hourly absorption of the resistors which feed the tunnel. Of course, one must in all cases add the costs of installation, maintenance and/or power requirements to the already high cost of the electric power. These power requirements are more or less reproduced as a fixed cost on each invoice from the power supplier. Furthermore, tunnel driers do not give optimal results in terms of the subsequent densification. This is due to the difficulty of the tunnel to optimally absorb the humidity lost by the material being dried.